When snowfall is heavy, the snow leopard is known to descend to lower

I also think A is the most appropriate. What is "whose" modifying in A?

IMO B is correct
Although A is also close.
eliminated it because of placement of also

Explanation from GMAT FREE, the source

Creating a filter: as we read the original sentence, we keep an eye on the placement of the word "also." But that placement could be correct, if the intended meaning is that the lower elevations, in addition to the upper elevations, have conditions favorable to the markor. We'll have to look for defects in the other choices and come back to (A).

Finding objective defects: choice (B) is defective in that "favorable" and "to" are separated by the word "conditions," violating the idiomatic usage of "favorable." Choice (B) is out. Choice (C) introduces a pronoun with no reference, "they," so it's out. Choice (D) has the same problem as (B), so it's out. Choice (E) has an idiomatic error, since "favorable for" is not proper usage. We are left with one choice. The correct answer is (A).

So in no circumstance are we allowed to put a word "condition" between "favourable" and "to?"

I have heard of split infinitives being considered an error, although I do not know whether necessarily because of bad grammar or because of poor style. Infinitive is = to + verb
Example: to sing merrily is ok,
to merrily sing is split infinitive – wrong
to bid aggressively is ok
to aggressively bid is split infinitive. – wrong.
But favorable …. to is not an infinitive. It is just a prepositional phrase. Prepositional phrase = preposition + a noun or noun phrase. I haven’t heard of any such error as a split prepositional phrase at least in GMAT.

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